A few studies have hinted that teas—with their bounty of antioxidants called polyphenols—might also exhibit antidiabetic properties. In the latest such trial, Lucy S. Hwang of National Taiwan University in Taipei measured green tea's effect on insulin action in rats with experimentally induced diabetes.

Hwang's team substituted room-temperature tea for drinking water for half of the animals. After 12 weeks, tea-drinking rodents exhibited improved insulin sensitivity and lower blood-glucose concentrations during the 2 hours after each meal, the researchers reported in the Feb. 1 Journal of Agricultural and Food Chemistry.

In related test-tube studies, the group measured how well fat cells from these animals absorb glucose, an action that in the body would lower blood sugar concentrations. The cells from diabetic rats drinking green tea absorbed more than twice as much of the sugar as did cells from similar animals drinking plain water—indicating, the researchers say, that the tea had indeed improved the fat cells' insulin sensitivity.

Hwang's group has now tested other types of tea. All true teas are made from leaves from the same species of plant. Green tea is unfermented, whereas black and other teas are fermented to various extents.

Like the green tea in the original test, semifermented pou-chong tea "significantly increased glucose uptake" by fat cells taken from diabetic animals that drank it, Hwang told Science News. However, fully fermented black tea—the favorite of most Western tea drinkers—didn't affect glucose absorption.

Since different teas contain different polyphenols that might underlie the fat-cell response, Hwang's team tested the antidiabetic effects of several polyphenols from the best-performing teas. The most effective turned out to be epigallocatechin gallate, an agent known to have anticancer properties (SN: 7/23/94, p. 61). In her lab tests, the compound has "insulinlike activity," Hwang says.

Hwang's team has traced the green tea's antidiabetic attributes to other mechanisms as well. In rats, green tea increased the number of insulin receptors on cells and the blood concentration of a protein—GLUT-IV—that helps move glucose out of the blood and into cells. Moreover, Hwang notes, the tea activated insulin-receptor kinase, an enzyme that makes the receptors available to bind insulin and initiate activity.